1. Field of the Invention
This invention is concerned with electrically-controlled hydraulic chirp-signal vibrators as used in seismic exploration. More specifically, this invention is concerned with automatically limiting the drive level of the vibrator to prevent electrical and/or mechanical damage to some of the components.
2. Discussion of the Prior Art
Typical servo-controlled hydraulic vibrators used to generate chirp signals for use in seismic exploration consist of a ground-contacting base plate to which is attached a vertical linear actuator shaft. A portion of the shaft is upset to form a piston. A reaction mass has a bore therethrough to receive the actuator shaft. A portion of the bore includes an enlarged cylindrical portion that forms an actuating chamber within which is fitted the piston of the linear actuator shaft. A port is provided at the upper and lower ends of the actuating chamber. A main servo valve provides pressurized hydraulic fluid through the upper or the lower port to apply hydraulic pressure alternately to opposite sides of the actuator piston thereby to shake the attached base plate with respect to the reaction mass.
A reference driver signal, through a DC servo amplifier drives the torque motor of a pilot servo valve. The pilot servo valve in turn drives the main servo valve to cause the actuator to shake the base plate in accordance with the desired chirp signal. A typical chirp signal might include a swept-frequency spectrum of 5 to 100 Hz. A manual drive-level adjustment is provided to adapt the vibrator to changing environmental conditions and variations in ground impedance.
It is known that it is preferable to maintain the output signal of the actuator in constant phase relation with the input driver signal. A feedback arrangement is provided to sense the output signal of the actuator and to multiply that signal by the driver signal and integrate the result to determine any change in phase between the two. A small phase change generates an error signal which, after integration is used to phase-shift the driver signal to restore the correct phase to the actuator output signal. See for example, U.S. Pat. No 3,219,971 to Cole which is incorporated herein by reference as a teaching of known art.
Feedback loops are provided between the actuator shaft and the main servo valve and the servo amplifier which provides the torque motor drive signal. The feedback loops stabilize operation of the servomechanical system.
The feedback signal from the main servo valve (valve feedback) and from the actuator (mass feedback) are often derived from linear variable differential transformers (LVDT). The input shafts of the LVDTs are coupled respectively to the main servo spool and the actuator shaft. The LVDTs sense the displacements and displacement directions of the servo spool and actuator shaft, the displacements being a function of the volume of fluid flow. The feedback voltages are fed through suitable demodulating amplifiers and thence to the DC servo amplifier. Manual adjustments may be provided in the feedback loops for stability adjustments. See for example, U.S. Pat. No. 3,881,167 to Pelton et al. and U.S. Pat. No. 4,184,144 to Rickenbacker, both of which are incorporated herein by reference. The drive level of the vibrator, that is, the force applied to the ground is typically manually adjusted by the operator.
We have found that for all practical purposes a vibrator is a band-limited signal generator. That is, at frequencies outside of a designated band, the vibrator is inherently incapable of generating a signal having an acceptable output level without damaging the component parts.
At very low frequencies, the actuator is stroke limited. Any attempt to increase the drive level results in an excessive displacement of the actuator shaft such that the piston strikes the upper and lower extremities of the actuator chamber, creating severe damage.
At very high frequencies the vibrator system is acceleration limited. The main servo valve cannot track the pilot valve signal because of the inertia of the main servo valve spool. As a result, the output signal level of the vibrator drops off. Again, an attempt to increase the drive level results in application of excessive current to the torque motor. The excessive current burns out the torque motor windings.
At low and intermediate frequencies, if the manual drive level adjustment is set too high, the spool of the servo valve hits the end stops. Severe valve damage occurs to the servo valve.
Thus, we have found that the manual drive-level adjustment is critically dependent upon the skill of the operator. If he sets the level too high, severe mechanical damage may result. Further, the drive level limit is sensitive to earth impedance. Changing conditions, as the vibrator moves from station to station may require frequent readjustments of the drive level which are sometimes overlooked by an unwary operator.